1. Field of the Invention
The present invention relates to an integrated circuit (IC) package, and more specifically to a method for attaching an IC package to a circuit board.
2. Description of the Prior Art
An integrated circuit is typically housed inside of a package made of ceramic or composite material. The package provides mechanical, electrical, and thermal protection to the IC chip. The package has contacts to which solder balls or pins are attached to allow the IC chip to be electrically connected to a circuit board, and at the same time to provide mechanical attachment of the package to the circuit board. The solder balls or pins serve the purpose of both electrical and mechanical connection.
FIG. 1 shows a bottom view of a prior art IC package 10 having an IC chip (item 16 of FIG. 2). A plurality of solder balls 12 is disposed on a bottom surface of the package 10 in a grid-like array for attaching the package to a circuit board.
A prior art method for attaching the IC package 10 to a circuit board 14 to manufacture an assembly 18 is illustrated in a side view shown in FIG. 2. The IC package 10 holding the IC chip 16 is attached to the circuit board 14 by the plurality of solder balls 12. During manufacture, the solder balls 12 are fused to both the package 10 and the circuit board 14 to form an electrical and mechanical connection.
The solder balls 12 can fail under stress from thermally induced mechanical loading or direct mechanical loading of the assembly 18. For instance, the package 10 and circuit board 14 typically have different coefficients of thermal expansion. As the assembly 18 is heated or cooled the circuit board 14 and package 10 expand or contract at different rates, and consequently the solder balls 12 can be subject to high stresses. Additionally, the circuit board 14 can be flexed by externally applied forces, such as those forces experienced during the manufacturing, assembly, and testing processes. However, because the mechanical stiffness of the IC package 10 is appreciably greater than that of the circuit board 14, the solder balls 12 can be subject to high stresses that can also cause mechanical failure. While the principal stresses in the solder balls 12 under these loading conditions may not be enough to cause immediate failure, a cyclic mechanical or thermal load can cause a fatigue failure.
FIG. 3 shows the assembly 18 bending about a single axis due to a differential thermal contraction of the circuit board 14 and the package 10. This condition can be caused, for example, by the assembly 18 being cooled from a higher than ambient assembly temperature resulting in the circuit-board 14 contracting more than the package 10. As the assembly 18 cooled, solder balls in two rows of solder balls 12a and 12b were subject to excessive tensile stress, and consequently cracked as shown. After a solder ball has failed mechanically, the IC package 10 is no longer effectively electrically connected to the circuit board 14. Furthermore, if the loading on the circuit board 14 is not somehow released, the package 10 may become completely mechanically separated from the circuit board 14.
The prior art method of using solder balls to provide a mechanical connection between an IC package and a circuit board results in a weak connection that is prone to mechanical failure. When the circuit board and package undergo a mechanical loading or thermally induced mechanical loading, solder balls can fail mechanically.